1. Field of the Invention
The present invention relates to correcting the process temperature in a multi-zone thermal processor. More specifically, the invention corrects for conditions affecting the measured process temperature for monitoring the heat convection process to a part in a multi-zone conveyorized thermal processor.
2. Background
Thermal processing involves a series of procedures by which an item may be exposed to a temperature-controlled environment, and is used in a variety of manufacturing procedures such as heat treating, quenching and refrigerated storage. One example of a thermal processor is a reflow oven. The production of various goods such as electronic circuit boards in solder reflow ovens frequently entails carefully controlled exposure to heating and/or cooling for specific periods. The elevated temperature conditions needed to solder component leads onto printed circuit boards must be gradually and uniformly applied to minimize thermal expansion stresses.
For this reason, convective heat transfer may be employed in these solder xe2x80x9creflowxe2x80x9d operations. The connecting solder paste incorporates an amalgam of substances that must undergo phase changes at separate temperature levels. Solder reflow may be performed by sequentially passing a part (such as a printed circuit board to become a processed product) through a series of thermally isolated adjacent regions or xe2x80x9czonesxe2x80x9d in the reflow oven, the temperature of each being independently adjusted. The part may be carried by a conveyor, which moves the part into the reflow oven entrance, through the zones, and out of the oven through the exit. The exposure of the part to the reflow oven conditions can be called a thermal process. A thermal process may performed on a part in a calibration run for measuring the part temperature on an instrumented part (in addition to setpoint and/or process temperatures) or a production run (for thermally processing printed circuit boards).
To conform a part""s temperature response to a target profile, setpoint temperatures and conveyor speed may be adjusted, and thereby correspond to a particular process temperature profile. Convection being an empirically characterized mode of heat transfer in a fluid medium depends on several factors, including the relative velocity between the medium and the part surface exposed and the temperature difference between the fluid medium (e.g., in the zone) Tz (in proximity to the part) and the physical part Tp. In a multi-zone reflow oven, thermal energy exchanged from the setpoint component (e.g., a heating element) to the part may be augmented by moving the fluid medium (typically air) situated therebetween. Moving the air in a zone may accelerate the transient response (thereby reducing the difference towards equilibrium between) the setpoint temperature Tc and the zone or process temperature Tz. This air motion may be produced by multi-blade fans rotating inside the zone at a given fan speed.
The process temperature may be measured by instruments (e.g., thermocouples) to monitor ambient conditions to the part. Typically, between thirty and sixty thermocouples may be evenly disposed along a probe in the oven to measure process temperatures. The series of process temperature measurements produce a process temperature profile. A part may also be instrumented for calibration purposes, but usually not in a production thermal process. The differences between measured process and part temperatures may be correlated for specified conditions. An instrument for correlation (e.g., a computer processor) may be used for this purpose. The rate of thermal energy convective exchange between the fluid medium and the part is proportional to their temperature differences. However, the process temperature may be influenced by collateral conditions in select regions of the reflow oven, thereby introducing potential errors in the correlated differences.
A conveyor may carry a part on rollers with axes disposed between two rails. This enables top and bottom surfaces of the part to be exposed to setpoint components above and below the part respectively, without a continuous support platform in between. When the conveyor carries a part from one zone to the next, the part may influence the air motion across the zone interface (perpendicular to the conveyor path). Air from one zone at a first temperature may be deflected by the part""s surface to the adjacent zone having a second temperature, causing undesirable mixing in the intruded adjacent zone. Such conditions may influence process temperature measurement, particularly for transition zones between heating and cooling portions of the reflow oven. A transition zone may represent a comparatively quiescent region with minimal air movement. Other zones with more vigorous fan rotation may be less influenced by such cross-flow.
As a part passes from an upstream adjacent zone to a transition zone, the part""s surface may deflect air from the upstream adjacent zone into the transition zone. Similarly, as the part is conveyed from the transition zone to a downstream adjacent zone, air from the downstream adjacent zone may move along the part into the transition zone. Such invasive air movement may alter its measured temperature to a value between the typical transition zone average and the influencing air from the adjacent zone. The resulting measured transient response may substantially influence the predicted part temperature profile without a similar corresponding effect on the actual part temperature profile.
Additionally, a part temperature profile may be predicted (for an established control setting) based on a calibration run with only a single instrumented part passing through the oven, while a production run may have a consecutive series of parts staggered in a sequence with uniform or nonuniform spacing therebetween. The measured process temperature in a transition zone may change depending on the quantity of and spacing between parts that facilitate air crossflow from adjacent zones. Such variation in process temperature response may increase the uncertainties associated with predicting the part temperature profile. Current methods lack an adequate means to correct for errors induced by cross-flow between zones. Hence, a method to obviate such errors is desired.
A method for adjusting process temperature measurements used to monitor zone conditions in a reflow oven selects values for exclusion from correlation with the predicted part temperature profile. During the thermal process, a plurality of process temperatures over intervals may be measured along the reflow oven. Over a time period for the thermal process, a measured process temperature of this plurality may be evaluated for the maximum and minimum extremities, from which a difference for that interval may be calculated. This difference may be compared to a qualifier for the plurality, and be excluded from part temperature prediction if the difference for the interval exceeds the qualifier.
Alternatively, the maximum and/or minimum extremities from the plurality of process temperature measurements may be incorporated to produce a process temperature profile for correlation with the part temperature profile. The incorporation of measurement extremities may substitute for time-integrated measurement values otherwise used for attaining a target part temperature response.